diff --git a/src/testsuite_support/mkdtemp_clean.c b/src/testsuite_support/mkdtemp_clean.c
index ad7729ff..f0d83b62 100644
--- a/src/testsuite_support/mkdtemp_clean.c
+++ b/src/testsuite_support/mkdtemp_clean.c
@@ -11,19 +11,28 @@
  * This file is the fix at the source. It provides a replacement `mkdtemp()`
  * that is linked into every unit-test executable in this project (via a
  * declare_dependency() appended to each testsuite's shared *_test_deps list,
- * see controller/meson.build and scheduler/testsuite/meson.build). Every
- * scratch dir it creates is registered and recursively removed by an
- * atexit() handler, so a test binary leaves /tmp clean regardless of which
- * test "owns" the dir. Cleanup runs at process exit (after ALL TEST_
- * functions), so tests that legitimately inspect a fixture's on-disk
- * contents across multiple TEST functions in the same file are unaffected --
- * the very property that made retrofitting cleanup into 70+ individual test
- * files invasive.
+ * see controller/meson.build and scheduler/testsuite/meson.build). Each
+ * scratch dir it creates is registered with the pid of the process that
+ * created it and removed at process exit (atexit) -- but ONLY by the process
+ * that created it.
+ *
+ * Per-entry owner pid is what makes this fork-safe without any pthread_atfork
+ * hook: atexit() handlers are inherited across fork(), so a forked child that
+ * later calls exit() would otherwise run the cleanup and delete every dir the
+ * PARENT registered -- including the dir the repl/sim suites' real-stdout-
+ * capture helper just wrote its capture file into, so the parent's subsequent
+ * fopen() sees ENOENT ("Expected Non-NULL" cascade). Filtering cleanup to
+ * entries whose owner pid == getpid() means a child's inherited atexit removes
+ * nothing the parent owns (parent pid != child pid), while dirs the child
+ * itself mkdtemp()s after the fork are owned by the child and cleaned at its
+ * own exit. No fork hooks are registered, so forking tests are completely
+ * unaffected (a global pthread_atfork was tried and regressed the HA/commit
+ * suites that fork heavily).
  *
  * Contract is glibc mkdtemp's: `tmpl` must end in XXXXXX (overwritten with a
  * unique directory name); returns `tmpl` on success, NULL on error. If the
- * dir was subsequently removed by the test itself, the atexit sweep is a
- * harmless no-op (rmdir fails with ENOENT).
+ * dir was subsequently removed by the test itself, the sweep is a harmless
+ * no-op (rmdir fails with ENOENT).
  */
 #define _GNU_SOURCE
 #include <dirent.h>
@@ -34,14 +43,20 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
+#include <sys/types.h>
 #include <time.h>
 #include <unistd.h>
 
 #define MKDTEMP_SUFFIX "XXXXXX"
 #define MKDTEMP_TRIES  1000
 
+typedef struct mkd_entry {
+    char*  path;
+    pid_t  owner;
+} mkd_entry_t;
+
 static pthread_mutex_t g_clean_mu       = PTHREAD_MUTEX_INITIALIZER;
-static char**          g_clean_dirs     = NULL;
+static mkd_entry_t*    g_clean_dirs     = NULL;
 static size_t          g_clean_count    = 0;
 static size_t          g_clean_capacity = 0;
 static int             g_clean_armed    = 0;
@@ -78,29 +93,34 @@ static void rmrf(const char* path)
     rmdir(path);
 }
 
+/* Remove every registered dir owned by THIS process. Forks inherit a copy of
+ * the parent's registry, but every entry carries the pid that created it, so
+ * a child's inherited atexit removes nothing the parent owns. */
 static void mkdtemp_cleanup_all(void)
 {
+    pid_t  me = getpid();
     size_t i;
 
     pthread_mutex_lock(&g_clean_mu);
     for (i = 0; i < g_clean_count; i++) {
-        if (g_clean_dirs[i] != NULL) {
-            rmrf(g_clean_dirs[i]);
-            free(g_clean_dirs[i]);
-            g_clean_dirs[i] = NULL;
+        if (g_clean_dirs[i].path != NULL && g_clean_dirs[i].owner == me) {
+            rmrf(g_clean_dirs[i].path);
+            free(g_clean_dirs[i].path);
+            g_clean_dirs[i].path  = NULL;
+            g_clean_dirs[i].owner = 0;
         }
     }
-    g_clean_count = 0;
     pthread_mutex_unlock(&g_clean_mu);
 }
 
 /* Own a copy of `path` (the caller's tmpl buffer is reused across mkdtemp
- * calls) and arrange for it to be removed at process exit (atexit). */
+ * calls), tagged with the creating pid, and arrange for it to be removed at
+ * process exit by that same pid. */
 static void track_dir(const char* path)
 {
-    char*  copy;
-    size_t ncap;
-    char** ndirs;
+    char*     copy;
+    size_t    ncap;
+    mkd_entry_t* ndirs;
 
     copy = strdup(path);
     if (copy == NULL) {
@@ -113,7 +133,7 @@ static void track_dir(const char* path)
     }
     if (g_clean_count == g_clean_capacity) {
         ncap  = g_clean_capacity ? g_clean_capacity * 2 : 64;
-        ndirs = (char**)realloc(g_clean_dirs, ncap * sizeof(*ndirs));
+        ndirs = (mkd_entry_t*)realloc(g_clean_dirs, ncap * sizeof(*ndirs));
         if (ndirs == NULL) {
             pthread_mutex_unlock(&g_clean_mu);
             free(copy);
@@ -122,7 +142,9 @@ static void track_dir(const char* path)
         g_clean_dirs     = ndirs;
         g_clean_capacity = ncap;
     }
-    g_clean_dirs[g_clean_count++] = copy;
+    g_clean_dirs[g_clean_count].path  = copy;
+    g_clean_dirs[g_clean_count].owner = getpid();
+    g_clean_count++;
     pthread_mutex_unlock(&g_clean_mu);
 }